The present invention relates to a hydraulic brake system, in particular for automotive vehicles, comprising at least one wheel bake adapted to be pressurized by a master brake cylinder, a hydraulic power booster for the actuation of the master brake cylinder, an auxiliary pressure which is adjustable in a pressure chamber of the hydraulic power booster by means of a brake valve, and a pressure-reducing valve interposed between the auxiliary pressure source and the brake valve controllable by the auxiliary pressure and having a valve which is adapted to be acted upon by the auxiliary pressure in the opening direction.
This type of brake system has been described in the prior German patent application P No. 32 18 194.9. The brake system according to this prior art comprises a tandem master cylinder which serves to pressurize two brake circuits. The tandem master cylinder is actuatable in turn by a hydraulic power booster which is substantially composed of a booster piston guided in a cylinder bore and of a brake valve arranged in the booster piston. The booster piston is provided with a circumferential groove at its outer peripheral surface so that an annular chamber will be provided in the hydraulic power booster which communicates via a pressure-reducing valve with an auxiliary pressure source. Upon application of the brake pedal, the brake valve will assume a position in which an auxiliary pressure is metered into the pressure chamber of the hydraulic power booster and displaces the booster piston in the cylinder bore of the hydraulic power booster upon attainment of a specific pressure level.
At the same time, the auxiliary pressure adjusted in the pressure chamber of the hydraulic power booster will be fed back to the pressure-reducing valve. As a result thereof the pressure at the outlet of the pressure-reducing valve and the pressure in the circumferential annular chamber of the booster piston will rise. Thus, the pressure-reducing valve serves to ensure that the pressure supplied to the booster piston will always be adjusted to the pressure requirement in the pressure chamber and to the actuating position. This will afford the advantage that the ring seals sealing the booster piston are not constantly exposed to the high pressure of the auxiliary pressure source so the high actuating forces and breakaway torques at the booster piston will be precluded to a large degree or at least reduced.
It has to be regarded at less favorable in the brake system described that the pressure-reducing valve is of relatively complicated construction. For example, the closure member of the pressure-reducing valve is part of a stepped piston which is guided in a stepped bore in a sealed relationship thereto. The reduction ratio of the pressure-reducing vale is predetermined by the dimensions of the effective surfaces of the stepped piston alone and cannot be adjusted retrospectively.